Incendivity of electrical sparking due to circulating currents in the enclosures of large electrical machines

An explosion occurred within an electric motor on an off-shore platform in late 1985. It is considered that this ignition could have been the result of electrical sparking at the joints of the motor enclosure, due to circulating currents. The authors show that where circulating currents are interrupted by discontinuities at joints in the current path, arcing or sparking can occur which may be incendive, despite the very low voltages involved. The energy available for ignition depends upon the inductance of the current path and the magnitude of the circulating current. Test results indicate that in machines of interest, the effective inductance of the current path is typically 1 μH to 2 μH